Recently, the housings of portable terminal devices typified by cellular phones, smart phones, personal digital assistants (PDAs), and tablet type terminals have been required to reduce the dimensions and weight from the viewpoint of compactness and light weightness. Accordingly, demands have arisen for more compact antenna devices. It has also been required to allow a single portable terminal device to communicate with a plurality of radio systems using different frequency bands.
Under the circumstances, for example, a folded monopole antenna has been proposed, which is obtained by folding the antenna element of a monopole antenna at a midway position so as to form a forward portion, a backward portion, and a ground point. There has also been proposed an antenna obtained by further folding the antenna element of this folded monopole antenna at a midway position. Using a multi-frequency folded monopole antenna using this folded structure can reduce a space required for mounting in a portable terminal device as compared with general folded antennas as well as general monopole antennas. Therefore, it can be expected to further reduce the sizes of portable terminal devices.
A folded monopole antenna using a folded structure obtains the first resonance in a frequency band in which the path length from the feeding point to the ground point through the forward and backward portions corresponds to almost ½ the wavelength of a general folded monopole antenna which does not use the folded structure, and the second resonance in a frequency band in which the path length from the feeding point to the ground point through the forward and backward portions corresponds to almost ⅔ the wavelength of the general folded monopole antenna. Of these resonant frequencies, the second resonant frequency may shift from the frequency band of a target radio system to result in a failure to communicate with the system.
A multi-frequency folded antenna has also been proposed, which is provided with the second antenna element in a direction opposite to the element direction of a folded monopole antenna. This type of antenna, however, is additionally provided with the second antenna element in the direction opposite to the monopole antenna element, and hence the total length of the antenna increases. This leads to an increase in the size of the antenna, which in turn becomes difficult to incorporate in a compact portable terminal device.